Coexpression of full-length gamma-aminobutyric acid(B) (GABA(B)) receptorswith truncated receptors and metabotropic glutamate receptor 4 supports the GABA(B) heterodimer as the functional receptor

Direct evidence is lacking to show whether the gamma-aminobutyric acid (GAB A)(B) gb1-gb2 heterodimer is the signaling form of the receptor. In this st udy, we tested whether gb1a or gb2 subunits when coexpressed with truncated receptors or metabotropic glutamate receptor mGluR4 could form functional GABA receptors. Coexpression of the ligand binding N-terminal domain of gb1 a or the C-terminal portion of gb1a composing the seven-transmembrane segme nts and intracellular loops with gb2 could not reconstitute functional rece ptors. We next examined whether mGluR4, which forms homodimers and is struc turally related to GABA(B), could act as a surrogate coreceptor for gb1 or gb2. The coexpression of mGluR4 and gb1a led to the expression of gb1a mono mers on cell surface membranes as determined by immunoblot analysis and flo w cytometry. However, mGluR4-gb1a heterodimers were not formed, and membran e-expressed gb1a monomers were not functionally coupled to adenylyl cyclase in human embryonic kidney 293 cells or activated inwardly rectifying potas sium (Kir) channels in Xenopus oocytes. Similarly, the coexpression of mGlu R4 and gb2 led to nonfunctional GABA receptors. GABA-activated distal signa ling events resulted only after the coexpression and heterodimerization of gb1 and gb2. Taken together with the truncated receptor studies, the data s uggest that a high degree of structural specificity is required to form the functional GABA(B) receptor that is a gb1-gb2 heterodimer.